Compact head-mounted display system having uniform image

ABSTRACT

There is disclosed an optical device, including a light-transmitting substrate having an input aperture, an output aperture, at least two major surfaces and edges, an optical element for coupling light waves into the substrate by total internal reflection, at least one partially reflecting surface located between the two major surfaces of the light-transmitting substrate for partially reflecting light waves out of the substrate, a first transparent plate, having at least two major surfaces, one of the major surfaces of the transparent plate being optically attached to a major surface of the light-transmitting substrate defining an interface plane, and a beam-splitting coating applied at the interface plane between the substrate and the transparent plate, wherein light waves coupled inside the light-transmitting substrate are partially reflected from the interface plane and partially pass therethrough.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andparticularly to devices which include a plurality of reflecting surfacescarried by a common light-transmissive substrate, also referred to as alight-guide optical element (LOE).

The invention can be implemented to advantage in a large number ofimaging applications, such as, for example, head-mounted and head-updisplays, cellular phones, compact displays, 3-D displays, compact beamexpanders as well as non-imaging applications such as flat-panelindicators, compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays, wherein an optical module serves both as animaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM) such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), or a scanning source andsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-sec-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. Unfortunately, as the desired field-of-view(FOV) of the system increases, such a conventional optical modulebecomes larger, heavier, bulkier, and therefore, even for a moderateperformance device, is impractical. This is a major drawback for allkinds of displays, but especially in head-mounted applications, whereinthe system must necessarily be as light and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on one hand, are still not sufficiently compactfor most practical applications, and, on the other hand, suffer majordrawbacks in terms of manufacturability. Furthermore, the eye-motion-box(EMB) of the optical viewing angles resulting from these designs isusually very small—typically less than 8 mm. Hence, the performance ofthe optical system is very sensitive, even to small movements of theoptical system relative to the eye of the viewer, and do not allowsufficient pupil motion for conveniently reading text from suchdisplays.

The teachings included in Publication Nos. WO01/95027, WO03/081320,WO2005/024485, WO2005/024491. WO2005/024969, WO2005/124427,WO2006/013565, WO2006/085309. WO2006/085310, WO2006/087709,WO2007/054928, WO2007/093983. WO2008/023367, WO2008/129539,WO2008/149339, WO2013/175465, IL 232197, IL 235642, IL 236490 and IL236491, all in the name of Applicant, are herein incorporated byreferences.

DISCLOSURE OF THE INVENTION

The present invention facilitates the design and fabrication of verycompact LOEs for, amongst other applications, head-mounted displays. Theinvention allows relatively wide FOVs together with relatively largeeye-motion-box values. The resulting optical system offers a large,high-quality image, which also accommodates large movements of the eye.The optical system offered by the present invention is particularlyadvantageous because it is substantially more compact thanstate-of-the-art implementations, and yet it can be readily incorporatedeven into optical systems having specialized configurations.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held applications such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the device of the end-user.The mobility requirement restricts the physical size of the displays,and the result is a direct-display with poor image viewing quality. Thepresent invention enables a physically very compact display with a verylarge virtual image. This is a key feature in mobile communications, andespecially for mobile internet access, solving one of the mainlimitations for its practical implementation. The present inventionthereby enables the viewing of the digital content of a full formatinternet page within a small, hand-held device, such as a cellularphone.

The broad object of the present invention is therefore to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

In accordance with the present invention there is therefore provided anoptical device, comprising a light-transmitting substrate having aninput aperture, an output aperture, at least two major surfaces andedges, an optical element for coupling light waves into the substrate bytotal internal reflection, at least one partially reflecting surfacelocated between the two major surfaces of the light-transmittingsubstrate for partially reflecting light waves out of the substrate, afirst transparent plate, having at least two major surfaces, one of themajor surfaces of the transparent plate being optically attached to amajor surface of the light-transmitting substrate defining an interfaceplane, and a beam-splitting coating applied at the interface planebetween the substrate and the transparent plate, wherein light wavescoupled inside the light-transmitting substrate are partially reflectedfrom the interface plane and partially pass therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of an exemplar prior art light-guide opticalelement;

FIGS. 2A and 2B are diagrams illustrating detailed sectional views of anexemplary array of selectively reflective surfaces;

FIG. 3 is a schematic sectional-view of a reflective surface with twodifferent impinging rays, according to the present invention;

FIG. 4 illustrates a sectional view of an exemplary array of selectivelyreflective surfaces wherein a transparent plate is attached to thesubstrate edge;

FIG. 5 is a schematic sectional-view of a reflective surface accordingto the present invention, illustrating the actual active aperture of thesurface;

FIG. 6 illustrates the active aperture size of the reflecting surfacesas a function of the field angle, for an exemplary LOE;

FIG. 7 illustrates detailed sectional views of the reflectance from anexemplary array of selectively reflective surfaces, for three differentviewing angles;

FIG. 8 illustrates the required distance between two adjacent reflectingsurfaces as a function of the field angle, for an exemplary LOE;

FIG. 9 is another schematic sectional-view of a reflective surface withtwo different impinging rays, according to the present invention;

FIG. 10 illustrates a sectional view of an exemplary array ofselectively reflective surfaces having a wedged transparent plate isattached to the substrate edge;

FIG. 11 is another schematic sectional-view of a reflective surface withtwo different impinging rays, according to the present invention,wherein the two rays are reflected from two partially reflectingsurfaces;

FIG. 12 is yet another schematic sectional-view of a reflective surfacewith two different impinging rays, according to the present invention,wherein the two rays are coupled into the LOE remotely located andcoupled-out of the LOE adjacent to each other;

FIGS. 13A and 13B are schematic sectional-views of a beam splittingsurface embedded inside a light-guide optical element;

FIG. 14 is a graph illustrating reflectance curves of a beam splittingsurface as a function of incident angles, for an exemplary angularsensitive coating for s-polarized light-waves;

FIG. 15 is a further graph illustrating reflectance curves of a beamsplitting surface as a function of incident angles, for an exemplaryangular sensitive coating for s-polarized light-waves;

FIG. 16 is a schematic sectional-view of two different beam splittingsurfaces embedded inside a light-guide optical element;

FIG. 17 is another schematic sectional-view of a beam splitting surfaceembedded inside a light-guide optical element wherein partiallyreflecting surfaces are fabricated inside the transparent attachedplate, and

FIGS. 18A and 18B are yet further schematic sectional-views ofembodiments of a beam-splitting surface embedded inside a light-guideoptical element wherein the coupling in, as well as the coupling-outelements are diffractive optical elements.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a sectional view of a light-guide optical element(LOE), according to the present invention. The first reflecting surface16 is illuminated by a collimated display 18 emanating from a lightsource (not shown) located behind the device. The reflecting surface 16reflects the incident light from the source such that the light istrapped inside a planar substrate 20 by total internal reflection. Afterseveral reflections off the surfaces 26, 27 of the substrate, thetrapped light waves reach an array of partially reflecting surfaces 22,which couple the light out of the substrate into the eye 24, having apupil 25, of a viewer. Herein, the input surface of the LOE will bedefined as the surface through which the input light waves enter the LOEand the output surface of the LOE will be defined as the surface throughwhich the trapped light waves exit the LOE. In addition, the inputaperture of the LOE will be referred to as the part of the input surfacethrough which the input light waves actually pass while entering theLOE, and the output aperture of the LOE will be referred to as a part ofthe output surface through which the output light waves actually passwhile exiting the LOE. In the case of the LOE illustrated in FIG. 1 ,both of the input and the output surfaces coincide with the lowersurface 26, however, other configurations are envisioned in which theinput and the image light waves could be located on opposite sides ofthe substrate, or on one of the edges of the LOE. Assuming that thecentral light wave of the source is coupled out of the substrate 20 in adirection normal to the substrate surface 26, the partially reflectingsurfaces 22 are flat, and the off-axis angle of the coupled light waveinside the substrate 20 is α_(in), then the angle α_(sur2) between thereflecting surfaces and the normal to the substrate plane is:

$\begin{matrix}{\alpha_{{sur}\; 2} = \frac{\alpha_{in}}{2}} & (1)\end{matrix}$

As can be seen in FIG. 1 , the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the partially reflecting surface22 from one of these directions 28 after an even number of reflectionsfrom the substrate surfaces 26 and 27, wherein the incident angle βrefbetween the trapped ray and the normal to the reflecting surface is:

$\begin{matrix}{\beta_{ref} = {{\alpha_{in} - \alpha_{{sur}\; 2}} = \frac{\alpha_{in}}{2}}} & (2)\end{matrix}$

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the substratesurfaces 26 and 27, where the off-axis angle is α′in =180°-αin and theincident angle between the trapped ray and the normal to the reflectingsurface is:

$\begin{matrix}{\beta_{ref}^{\prime} = {{\alpha_{in}^{\prime} - \alpha_{{sur}\; 2}} = {{{- \alpha_{in}} - \alpha_{{sur}\; 2}} = {- \frac{3\alpha_{in}}{2}}}}} & (3)\end{matrix}$wherein the minus sign denotes that the trapped ray impinges on theother side of the partially reflecting surface 22.

As illustrated in FIG. 1 , for each reflecting surface, each ray firstarrives at the surface from the direction 30, wherein some of the raysagain impinge on the surface from direction 28. In order to preventundesired reflections and ghost images, it is important that thereflectance be negligible for the rays that impinge on the surfacehaving the second direction 28.

An important issue that must be considered is the actual active area ofeach reflecting surface. A potential non-uniformity in the resultingimage might occur due to the different reflection sequences of differentrays that reach each selectively reflecting surface: some rays arrivewithout previous interaction with a selectively reflecting surface;other rays arrive after one or more partial reflections. This effect isillustrated in FIG. 2A. Assuming that, for example, α_(in)=50°, the ray80 intersects the first partially reflecting surface 22 at point 82. Theincident angle of the ray is 25° and a portion of the ray's energy iscoupled out of the substrate. The ray then intersects the sameselectively partially reflecting surface at point 84 at an incidentangle of 75° without noticeable reflection, and then intersects again atpoint 86 at an incident angle of 25°, where another portion of theenergy of the ray is coupled out of the substrate. In contrast, the ray88 shown in FIG. 2B, experiences only one reflection 90 from the samesurface. Further multiple reflections occur at other partiallyreflecting surfaces.

FIG. 3 illustrates this non-uniformity phenomenon with a detailedsectional view of the partially reflective surface 22, which coupleslight trapped inside the substrate out and into the eye 24 of a viewer.As can be seen, the ray 80 is reflected off the upper surface 27, nextto the line 100, which is the intersection of the reflecting surface 22with the upper surface 27. Since this ray does not impinge on thereflecting surface 22, its brightness remains the same and its firstincidence at surface 22 is at the point 102, after double reflectionfrom both external surfaces. At this point, the light wave is partiallyreflected and the ray 104 is coupled out of the substrate 20. For otherrays, such as ray 88, which is located just below ray 80, the firstincidence at surface 22 is before it meets the upper surface 27, atpoint 106 wherein the light wave is partially reflected and the ray 108is coupled out of the substrate. Hence, when it again impinges onsurface 22, at point 110 following double reflection from the externalsurfaces 26, 27, the brightness of the coupled-out ray is lower than theadjacent ray 104. As a result, all the rays with the same coupled-inangle as 80 that arrive at surface 22 left of the point 102 have lowerbrightness. Consequently, the reflectance from surface 22 is actually“darker” left of the point 102 for this particular couple-in angle.

It is difficult to fully compensate for such differences inmultiple-intersection effects nevertheless, in practice, the human eyetolerates significant variations in brightness, which remain unnoticed.For near-to-eye displays, the eye integrates the light which emergesfrom a single viewing angle and focuses it onto one point on the retina,and since the response curve of the eye is logarithmic, smallvariations, if any, in the brightness of the display will not benoticeable. Therefore, even for moderate levels of illuminationuniformity within the display, the human eye experiences a high-qualityimage. The required moderate uniformity can readily be achieved with theelement illustrated in FIG. 1 . For systems having large FOVs, and wherea large EMB is required, a comparatively large number of partiallyreflecting surfaces is required, to achieve the desired output aperture.As a result, the non-uniformity due to the multiple intersections withthe large number of partially reflecting surfaces becomes more dominant,especially for displays located at a distance from the eye, such ashead-up displays and the non-uniformity cannot be accepted. For thesecases, a more systematic method to overcome the non-uniformity isrequired.

Since the “darker” portions of the partially reflecting surfaces 22contribute less to the coupling of the trapped light waves out of thesubstrate, their impact on the optical performance of the LOE can beonly be negative, namely, there will be darker portions in the outputaperture of the system and dark stripes will exist in the image. Thetransparency of each one of the reflecting surfaces is, however, uniformwith respect to the light waves from the external scene. Therefore, ifoverlapping is set between the reflective surfaces to compensate for thedarker portions in the output aperture, then rays from the output scenethat cross these overlapped areas will suffer from double attenuations,and darker stripes will be created in the external scene. Thisphenomenon significantly reduces the performance not only of displayswhich are located at a distance from the eye, such as head-up displays,but also that of near-eye displays, and hence, it cannot be utilized.

FIG. 4 illustrates an embodiment for overcoming this problem. Only the“bright” portions of the partially reflecting surfaces 22 a, 22 b and 22c are embedded inside the substrate, namely, the reflecting surfaces 22a, 22 b and 22 c no longer intersect with the lower major surface 26,but terminate short of this surface. Since the ends of the reflectingsurfaces are adjacent to one another over the length of the LOE, therewill be no gaps in the projected image, and since there is no overlapbetween the surfaces there will be no gaps in the external view. Thereare several ways to construct this LOE, one of which is to attach atransparent plate 120 having a thickness T, preferably by opticalcementing, to the active area of the substrate. In order to utilize onlythe active areas of the reflective surfaces 22 in the correct manner, itis important to calculate the actual active area of each partiallyreflective surface and the required thickness T of the plate 120.

As illustrated in FIG. 5 , the bright aperture D_(n) of the reflectivesurface 22 n in the plane of the external surface 26, as a function ofthe coupled-in angle α_(in), is:

$\begin{matrix}{D_{n} = \frac{2d}{{\cot\left( \alpha_{sur} \right)} + {\cot\left( \alpha_{in} \right)}}} & (4)\end{matrix}$

Since the trapped angle α_(in) can be varied as a function of the FOV,it is important to know with which angle to associate each reflectingsurface 22 n, in order to calculate its active aperture.

FIG. 6 illustrates the active aperture as a function of the field anglefor the following system parameters; substrate thickness d=2 mm,substrate refractive index v=1.51, and partially reflecting surfaceangle α_(sur)=64°. In consideration of the viewing angles, it is notedthat different portions of the resulting image originate from differentportions of the partially reflecting surfaces.

FIG. 7 , which is a sectional view of a compact LOE display system basedon the proposed configuration, illustrates this effect. Here, a singleplane light wave 112, representing a particular viewing angle 114,illuminates only part of the overall array of partially reflectingsurfaces 22 a, 22 b and 22 c. Thus, for each point on the partiallyreflecting surface, a nominal viewing angle is defined, and the requiredactive area of the reflecting surface is calculated according to thisangle. The exact, detailed design of the active area of the variouspartially reflective surfaces is performed as follows: for eachparticular surface, a ray is plotted (taking refraction, due to Snell'sLaw, into consideration) from the left edge of the surface to the centerof the designated eye pupil 25. The calculated direction is set as thenominal incident direction and the particular active area is calculatedaccording to that direction.

As seen in FIG. 5 , the exact values of the reflecting surfaces activeareas can be used to determine the various distances T between the leftedge 102 of the bright part of each reflecting surface 22 _(n) and thelower surface 26. A larger active area dictates a smaller inter-surfacedistance. This distance represents the thickness of the plate 120 (FIG.7 ) that should be attached to the lower surface of the LOE. Asillustrated in FIG. 5 , the distance T as a function of the coupled-inangle α_(in), is:T=d−D _(n)·cot(α_(sur))  (5)

FIG. 8 illustrates the required thickness T of the plate 120 as afunction of the field angle, for the same parameters as set above inreference to FIG. 6 . It is worthwhile setting the thickness T as themaximal calculated value to assure that the phenomenon of dark stripeswill be avoided in the image. Setting a too thick plate 120 will causean opposite effect, namely, the appearance of bright stripes in theimage.

As illustrated in FIG. 9 , two light rays, 122 and 124, are coupledinside the substrate 20. The two rays are partially reflected fromsurface 22 a at points 126 and 128, respectively. Only ray 122, however,impinges on the second surface 22 b at point 130 and is partiallyreflected there, while ray 124 skips over surface 22 b without anyreflectance. As a result, the brightness of ray 124, which impinges onsurface 22 c at point 134, is higher than that of ray 122 at point 132.Therefore, the brightness of the coupled-out ray 138 from point 134 ishigher than that of ray 136 which is coupled-out from point 132, and abright stripe will appear in the image. Consequently, an exact value ofthe thickness T should be chosen to avoid dark as well as bright stripesin the image.

As illustrated in FIG. 10 , a possible embodiment for achieving therequired structure, wherein the thickness T of the plate 120 depends onthe viewing angle, is to construct a wedged substrate 20′, wherein thetwo major surfaces are not parallel. A complementary transparent wedgedplate 120′ is attached to the substrate, preferably by opticalcementing, in such a way that the combined structure forms a completerectangular parallelepiped, i.e., the two outer major surfaces of thefinal LOE are parallel to each other. There are, however, some drawbacksto this method. First of all, the fabrication process of the wedged LOEis more complicated and cumbersome than the parallel one. In addition,this solution is efficient for systems having small EMB, wherein thereis a good matching between the viewing angle and the lateral position onthe substrate plane. For systems having a large EMB, however, namely,wherein the eye can move significantly along the lateral axis, therewill be no good adjustment between the viewing angle and the actualthickness of the plate 120′. Hence, dark or bright stripes may be seenin the image.

This occurrence of dark or bright stripes due to the structure of thepartially reflective surfaces in the LOE is not limited to the surfacewhich creates this phenomenon. As illustrated with reference to FIG. 3 ,the brightness of the coupled ray 88, which is reflected twice bysurface 22 a, is lower at point 110 than that of ray 80, which isreflected only once from surface 22 a at point 102. As a result, thebrightness of the reflected wave 112 is lower than that of the adjacentray 104. As illustrated in FIG. 11 , however, not only the brightness ofthe reflected wave from surface 22 a is different, but also thebrightness of the transmitted rays 140 and 142 is different. As aresult, the brightness of the reflected rays 144 and 146 from surface 22b, at points 148 and 150, respectively, will be different in the sameway and a dark stripe will be created also at this region of the image,as well. Naturally, this dissimilarity between the rays will continue topropagate in the LOE to the next partially reflective surfaces. As aresult, since each partially reflective surface creates its own dark orbright stripes, according to the exact incident angle, for an LOE havinga large number of partially reflecting surfaces, a large amount of darkand bright stripes will be accumulated at the far edge of the outputaperture of the LOE, and consequently, the image quality will beseverely deteriorated.

Another source for unevenness of the image can be the non-uniformity ofthe image waves which are coupled into the LOE. Usually, when two edgesof a light source have slightly different intensities this will hardlybe noticed by the viewer, if at all. This situation is completelydifferent for an image which is coupled inside a substrate and graduallycoupled-out, like in the LOE. As illustrated in FIG. 12 , two rays 152and 154 are located at the edges of the plane wave 156, which originatesfrom the same point in the display source (not shown). Assuming that thebrightness of ray 152 is lower than that of ray 154 as a result of anon-perfect imaging system, this non equality will hardly be seen bydirect viewing of the plane wave 156 because of the remoteness betweenthe rays. However, after being coupled into the LOE 20, this conditionis changed. While the ray 154 illuminates the reflecting surface 16 justright to the interface line 156 between the reflecting surface 16 andthe lower major surface 26, the right ray 152 is reflected from surface16, totally reflected from the upper surface 27, and then impinges onthe lower surface 26 just left to the interface line 158. As a result,the two rays 152 and 154 propagate inside the LOE 20 adjacent to eachother. The two exit rays 160 and 162, which originated from rays 152 and154, respectively, and reflected from surface 22 a, have accordinglydifferent brightness. Unlike the input light wave 156, however, the twodifferent rays are adjacent to each other, and this dissimilarity willeasily be seen as a dark stripe in the image. These two rays 164, 165will continue to propagate together, adjacent to each other, inside theLOE and will create a dark stripe at each place that they will becoupled out together. Naturally, the best way to avoid this unevennessis to assure that all the coupled light waves into the LOE have auniform brightness over the entire input aperture for the entire FOV.This demand might be very difficult to fulfil for systems having largeFOV as well as wide input apertures.

As illustrated in FIGS. 13A and 13B, this unevenness problem may besolved by attaching a transparent plate to one of the major surfaces ofthe LOE, as described above with reference to FIG. 4 . In thisembodiment, however, a beam splitting coating 166 is applied to theinterface plane 167 between the LOE 20 and the transparent plate 120. Asillustrated in FIG. 13A, two light rays, 168 and 170, are coupled insidethe substrate 20. Only ray 168 impinges on the first partiallyreflective surface 22 a at point 172 and is partially reflected there,while ray 170 skips over surface 22 a, without any reflectance. As aresult, assuming that the two rays have the same brightness whilecoupled into the LOE, ray 170 which is reflected upward from the lowermajor surface 26 has a higher brightness then ray 168 which is reflecteddownward from the upper surface 27. These two rays intersect each otherat point 174, which is located at the interface plane 167. Due to thebeam splitting coating which is applied thereto, each one of the twointersecting rays is partially reflected and partially passes throughthe coating. Consequently, the two rays interchange energies betweenthemselves and the emerging rays 176 and 178 from the intersection point174 have a similar brightness, which is substantially the averagebrightness of the two incident rays 168 and 170. In addition, the raysexchange energies with two other rays (not shown) at intersection points180 and 182. As a result of this energy exchange, the two reflected rays184 and 186 from surface 22 b will have substantially similar brightnessand the bright stripe effect will be significantly improved.

Similarly, as illustrated in FIG. 13B, two light rays, 188 and 190, arecoupled inside the substrate 20. Only ray 188, however, impinges on thefirst partially reflective surface 22 a at point 192 and partiallyreflected there before being reflected by the upper surface 27. As aresult, assuming that the two rays have the same brightness whilecoupled into the LOE, ray 190 which is reflected downward from the uppermajor surface 27, has a higher brightness then ray 188. These two rays,however, intersect each other at point 194 which is located at theinterface plane 167 and exchange energies there. In addition, these tworays intersect with other rays at the points 196 and 198 which arelocated on the beam splitting surface 167. As a result, the rays 200 and202 which are reflected from surface 22 a and consequentially the rays204 and 206 which are reflected from surface 22 b, will havesubstantially the same brightness, and therefore, the dark stripeseffect will be significantly decreased. This improved uniformity ofbrightness effect is applicable also for dark and bright stripes, whichare caused by a non-uniform illumination at the input aperture of theLOE. As a result, the brightness distribution of the optical waves,which is trapped inside the LOE, is substantially more uniform over theoutput aperture of the LOE than over the input aperture.

As illustrated in FIG. 13A the light rays 184, 186, which are reflectedfrom surface 22 a, intersect with the beam splitting surface 167, beforebeing coupled out from the LOE. As a result, a simple reflecting coatingcannot be easily applied to surface 167 since this surface should alsobe transparent to the light-waves that exit the substrate 20 as well astransparent to the light wave from the external scene for see-throughapplications, namely, the light-waves should pass through plane 167 atsmall incident angles, and be partially reflected at higher incidentangles. Usually, the passing incident angles are between 0° and 15° andthe partially reflecting incident angles are between 40° and 65°. Inaddition, since the light rays cross the interface surface 167 manytimes while propagating inside the LOE, the absorption of the coatingshould be negligible. As a result, a simple metallic coating cannot beused and a dielectric thin-film coating, having a high transparency hasto be utilized.

FIG. 14 illustrates for s-polarization the reflectance curves asfunctions of the incident angles for three representative wavelengths inthe photopic region: 470 nm, 550 nm and 630 nm. As illustrated, it ispossible to achieve the required behavior of partial reflectance(between 45% and 55%) at large incident angles between 40° and 65° andlow reflectance (below 5%) at small incident angles, for s-polarizedlight-waves. For p-polarized light-waves, it is impossible to achievesubstantial reflectance at incident angles between 40° and 65°, due tothe proximity to the Brewster angle. Since the polarization which isusually utilized for an LOE-based imaging system, is the s-polarization,the required beam splitter can be fairly easily applied. However, sincethe beam splitting coating should be substantially transparent for lightwaves from the external scene which impinge on the interface surface atlow incident angles and which are substantially non-polarized, thecoating should have low reflectance (below 5%) at small incident anglesalso for p-polarized light waves.

A difficulty still existing is that the LOE 20 is assembled from severaldifferent components. Since the fabrication process usually involvescementing optical elements, and since the required angular-sensitivereflecting coating is applied to the light-guide surface only after thebody of the LOE 20 is complete, it is not possible to utilize theconventional hot-coating procedures that may damage the cemented areas.Novel thin-film technologies, as well as ion-assisted coatingprocedures, can also be used for cold processing. Eliminating the needto heat parts, allows cemented parts to be safely coated. An alternativeis that the required coating can simply be applied to transparent plate120, which is adjacent to the LOE 20, utilizing conventional hot-coatingprocedures and then cementing it at the proper place. Clearly, hisalternative approach can be utilized only if the transparent plate 120is not too thin and hence might be deformed during the coating process.

There are some issues that should be taken into consideration whiledesigning a beam splitting mechanism as illustrate above:

-   -   a. Since the rays which are trapped inside the LOE are not only        totally reflected from the major surfaces 26 and 27, but also        from the internal partially reflecting interface plane 167, it        is important that all three of these surfaces will be parallel        to each other to ensure that coupled rays will retain their        original coupling-in direction inside the LOE.    -   b. As illustrated in FIGS. 13A and 13B, the transparent plate        120 is thinner than the original LOE 20. Unlike the        considerations which were brought regarding to the uncoated        plate in FIGS. 7-10 , wherein the thickness of plate 120 is        important for uniformity optimization, however, here the        thickness of the coated plate might be chosen according to other        considerations. On one hand, it is easier to fabricate, coat and        cement a thicker plate. On the other hand with a thinner plate        the effective volume of the LOE 20, which is practically coupled        the light waves out of the substrate, is higher for a given        substrate thickness. In addition, the exact ratio between the        thicknesses of the plate 120 and the LOE 20 might influence the        energy interchange process inside the substrate.    -   c. Usually, for beam splitters which are designated for full        color images the reflectance curve should be as uniform as        possible for the entire photopic region, in order to abort        chromatic effects. Since, however, in the configurations which        are illustrated in the present invention the various rays        intersect with each other many times before being coupled out        from the LOE 20, this requirement is no longer essential.        Naturally, the beam-splitting coating should take into account        the entire wavelengths spectrum of the coupled image, but the        chromatic flatness of the partially reflecting curve may be        tolerated according to various parameters of the system.    -   d. The reflectance-transmittance ratio of the beam-splitting        coating should not necessarily be 50%-50%. Other ratios may be        utilized in order to achieve the required energies exchange        between the darker and the brighter rays. Moreover, as        illustrated in FIG. 15 , a simpler beam-splitter coating can be        utilized, wherein the reflectance is gradually increased from        35% at an incident angle of 40° to 60% at an incident angle of        65°.    -   e. The number of the beam-splitting surfaces which are added to        the LOE is not limited to one. As illustrated in FIG. 16 ,        another transparent plate 208 may be cemented to the upper        surface of the LOE, wherein a similar beam-splitting coating is        applied to the interface plane 210 between the LOE 20 and the        upper plate 208, to form an optical device with two beam        splitting surfaces. Here, the two unequal rays 212 and 214        intersect with each other at point 215 on the coated interface        plane 210 along with other intersections with other rays at        points 216 and 217. This is in addition to the intersections on        the lower beam-splitting interface plane 167. As a result, it is        expected that the uniformity of the reflected rays 218 and 220        will be even better than that of the embodiments of FIGS. 13A        and 13B. Naturally, the fabrication method of the LOE having two        beam-splitting interface planes is more difficult than that of        having only a single plane. Therefore, it should be considered        only for systems wherein the non-uniformity problem is severe.        As before, it is important that all of the four reflecting        surfaces and planes 26, 27, 167 and 210, should be parallel to        each other.    -   f. The transparent plate 120 should not be necessarily        fabricated from the same optical material as the LOE 20.        Furthermore, the LOE might be fabricated of a silicate based        material while, for the sake of eye safety, the transparent        layer may be fabricated of a polymer based material. Naturally,        care should be taken to ensure optical qualities of the external        surfaces and to avoid deformation of the transparent plate.    -   g. So far it was assumed that the transparent plate is totally        blank. However, as illustrated in FIG. 17 , partially reflecting        surfaces 222 a and 222 b, may be fabricated inside the plate        120, in order to increase the useable volume of the LOE. These        surfaces should be strictly parallel to the existing surfaces 22        a and 22 b and oriented at exactly the same orientation.

All the various parameters of the above embodiments, such as, thethickness and the optical material of the plate 120, the exact nature ofthe beam-splitting coating, the number of the beam-splitting surfacesand location of the partially reflecting surface inside the LOE, couldhave many different possible values. The exact values of these factorsare determined according to the various parameters of the optical systemas well as the specific requirements for optical quality and fabricationcosts.

So far, it was assumed that the light waves are coupled out from thesubstrate by partially reflecting surfaces, which are oriented at anoblique angle in relation to the major surfaces, and usually coated witha dielectric coating. As illustrated in FIG. 18A, however, there aresystems wherein the light waves are coupled into and out from thesubstrate utilizing diffractive elements 230 and 232, respectively. Thesame uniformity issues that were discussed above should also be relevantto this configuration. As illustrated, the two rays 234 and 236 from thesame point in the display source are coupled into the substrate 238remotely located from each other at the two edges of the coupling-inelement 230. The rays are coupled-out by the coupling-out element 232located adjacent to each other. Therefore, any dissimilarity between therays will be easily seen in the coupled-out wave. In addition, in orderto validate a uniformed coupled-out image the diffractive efficiency ofthe coupling-out element 232 is increased gradually. As a result,different rays from the same point source might pass through differentlocations in the element 232 before being coupled-out the element andhence will have different brightness in the image. Another source forthe unevenness can be caused by the fact that the ray 234 is partiallydiffracted out of the substrate at the right edge 240 of the grating 232while ray 236 impinges on the lower surface just left of the grating,and hence, is not diffracted there. As a result, for all thecoupling-out locations in the grating 232 for the two adjacent rays 234and 236, ray 236 will have a higher brightness and this difference willeasily be seen.

FIG. 18B illustrates a similar approach to solve these issues. Asillustrated, a transparent plate 242 is cemented to the upper surface244 of the substrate 238, wherein the interface surface 246 is coatedwith a beam-splitting coating similar to the above-described coatings.

The invention claimed is:
 1. An optical device, comprising: alight-guide optical element having two mutually-parallel major surfacesand edges, an input aperture through which light waves enter thelight-guide optical element and an output aperture through which thelight waves exit the light-guide optical element; at least onepartially-reflecting surface for coupling-out the light waves from oneof the major surfaces of the light-guide optical element at said outputaperture, said at least one partially-reflecting surface being locatedbetween, and oriented obliquely to, said two major surfaces of thelight-guide optical element; and at least one beam-splitting surfaceembedded inside the light-guide optical element between, separated from,and parallel to, said two major surfaces of the light-guide opticalelement, said at least one beam-splitting surface being implemented as adielectric thin-film coating configured to be partially reflecting andpartially transmitting with a reflectance of between 35% and 60% fors-polarized light of each of at least three wavelengths of visible lightfor all incident angles within a range spanning at least 20°, said rangebeing at angles over 40° to a normal to said beam-splitting surface, anda reflectance of less than 10% for light of said at least threewavelengths of visible light incident at angles of less than 15° to thenormal to said beam-splitting surface, said at least three wavelengthsincluding wavelengths of 470, 550 and 630 nm, and wherein the lightwaves propagating by total internal reflection at said major surfacesundergo a plurality of total internal reflections at said two majorsurfaces and impinge a plurality of times on said at least onebeam-splitting surface between said input aperture and said outputaperture.
 2. The optical device according to claim 1, wherein thereflectance of the dielectric thin-film coating varies by no more than10 percent of its value for each of said at least three wavelengths ofvisible light across incident angles spanning said range of incidentangles.
 3. The optical device according to claim 1, wherein thereflectance of the of the dielectric thin-film coating increases as afunction of the incident angle across incident angles spanning saidrange of incident angles.
 4. The optical device according to claim 1,wherein the reflectance percentage of the beam-splitting coating differsby no more than 10 between said at least three wavelengths of visiblelight for each incident angle within said range of incident angles. 5.The optical device according to claim 1, further comprising an opticalelement for coupling the light waves into said light-guide opticalelement.
 6. The optical device according to claim 1, wherein said atleast one partially reflecting surface is coated with a dielectriccoating.
 7. The optical device according to claim 1, wherein said atleast one partially-reflecting surface for coupling-out the light wavesincludes a first partially-reflecting surface on one side of saidbeam-splitting surface and a second partially-reflecting surface on asecond side of said beam-splitting surface.
 8. The optical deviceaccording to claim 1, wherein said at least one beam-splitting surfaceat least partially overlaps said output aperture.
 9. The optical deviceaccording to claim 1, wherein said light-guide optical element isfabricated of two different optical materials.
 10. The optical deviceaccording to claim 9, wherein said two different optical materialsincludes a silicate-based material and a polymer-based material.
 11. Theoptical device according to claim 1, wherein said at least onebeam-splitting surface is implemented as at least two beam-splittingsurfaces, each embedded inside the light-guide optical element between,and parallel to, said two major surfaces of the light-guide opticalelement.
 12. The optical device according to claim 11, wherein said atleast one partially-reflecting surface for coupling-out the light wavesis interposed between two of said beam-splitting surfaces.